My wife was awakened at night by a really loud sound coming from our furnace (Trane XE 78). We have a light switch that turns off our furnace. She hit the switch and turned it off. Is there any danger of gas, carbon monoxide, or any other fumes coming out when the furnace is off? I have a couple kids sleeping not far from the furnace and am concerned

Once the power is shut off there is very little to worry about. In you case I'd advise that you have a professional HVAC person come out and diagnos the problem.